The invention relates to digital television (DTV) signals for over-the-air broadcasting, transmitters for such broadcast DTV signals, and receivers for such broadcast DTV signals, which broadcast DTV signals include novel echo-cancellation reference (ECR) signal components or initializing the parameters of adaptive filters used in the DTV receivers for channel-equalization and echo-cancellation.
The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as xe2x80x9cA/53xe2x80x9d for sake of brevity. Annex D of A/53 titled xe2x80x9cRF/Transmission Systems Characteristicsxe2x80x9d is particularly incorporated by reference into this specification. Annex D specifies that the data frame shall be composed of two data fields, each data field composed of 313 data segments, and each data segment composed of 832 symbols. Annex D specifies that each data segment shall begin with a 4-symbol data-segment-synchronization (DSS) sequence. Annex D specifies that the initial data segment of each data field shall contain a data-field synchronization (DFS) signal following the 4-symbol DSS sequence therein. The fifth through 515th symbols in each A/53 DFS signal are a specified PN511 sequencexe2x80x94that is a pseudo-random noise sequence composed of 511 symbols capable of being rendered as +5 or xe2x88x925 values. The 516th through 704th symbols in each A/53 DFS signal are a triple-PN63 sequence composed of a total of 189 symbols capable of being rendered as +5 or xe2x88x925 values. The middle PN63 sequence is inverted in polarity every other data field. The 705th through 728th symbols in each A/53 DFS signal contain a VSB mode code specifying the nature of the vestigial-sideband (VSB) signal being transmitted. The remaining 104 symbols in the each A/53 DFS signal are reserved, with the last twelve of these symbols being a precode signal that repeats the last twelve symbols of the data in the last data segment of the previous data field. A/53 specifies such precode signal to implement trellis coding and decoding procedures being able to resume in the second data segment of each field proceeding from where those procedures left off processing the data in the preceding data field.
The broadcast TV signal to which the receiver synchronizes its operations is called the principal signal, and the principal signal is usually the direct signal received over the shortest transmission path. Thus, the multipath signals received over other paths are usually delayed with respect to the principal signal and appear as lagging ghost signals. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes its operations to a (longer path) signal that is delayed respective to the direct signal, there will be a leading multipath signal caused by the direct signal, or there will a plurality of leading multipath signals caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. In the analog TV art multipath signals are referred to as xe2x80x9cghostsxe2x80x9d, but in the DTV art multipath signals are customarily referred to as xe2x80x9cechoesxe2x80x9d. The multipath signals that lead the principal signal are referred to as xe2x80x9cpre-echoesxe2x80x9d, and the multipath signals that lag the principal signal are referred to as xe2x80x9cpost-echoesxe2x80x9d. The echoes vary in number, amplitude and delay time from location to location and from channel to channel at a given location. Post-echoes with significant energy have been reported as being delayed from the reference signal by as many as sixty microseconds. Pre-echoes with significant energy have been reported leading the reference signal by as many as thirty microseconds. This 90-microsecond or so possible range of echoes of is appreciably more extensive than was generally supposed before spring 2000.
The transmission of the digital television (DTV) signal to the receiver is considered to be through a transmission channel that has the characteristics of a sampled-data time-domain filter that provides weighted summation of variously delayed responses to the transmitted signal. In the DTV signal receiver the received signal is passed through equalization and echo-cancellation filtering that compensates at least partially for the time-domain filtering effects that originate in the transmission channel. This equalization and echo-cancellation filtering is customarily sampled-data filtering performed in the digital domain. Time-domain filtering effects differ for the channels through which broadcast digital television signals are received from various transmitters. Furthermore, time-domain filtering effects change over time for the broadcast digital television signals received from each particular transmitter. Changes referred to as xe2x80x9cdynamic multipathxe2x80x9d are introduced while receiving from a single transmitter when the lengths of reflective transmission paths change, owing to the reflections being from moving objects. Accordingly adaptive filter procedures are required for adjusting the weighting coefficients of the sampled-data filtering that provides echo-cancellation and equalization.
Determination of the weighting coefficients of the sampled-data filtering that provides equalization and echo-cancellation is customarily attempted using a method of one of two general types. A method of the first general type relies on analysis of the effects of multipath just on an echo-cancellation reference (ECR) signal included in the transmitted signal specifically to facilitate such analysis. A method of the second general type relies on analysis of the effects of multipath on all portions of the transmitted signal. While the PN511 and triple-PN63 sequences in the initial data segments of the data fields in the ATSC standard DTV signal were originally proposed for use as ECR signals, the VSB receiver performance in actual field environments has demonstrated that these sequences are inadequate ECR signals, considered separately or in combination. So, most DTV manufacturers have used decision-feedback methods that rely on analysis of the effects of multipath on all portions of the transmitted signal for adapting the weighting coefficients of the sampled-data filtering. Decision-feedback methods that utilize least-mean-squares (LMS) method or block LMS method can be implemented in an integrated circuit of reasonable size. These decision-feedback methods provide for tracking dynamic multipath conditions reasonably well after the equalization and echo-cancellation filtering has initially been converged to substantially optimal response, providing that the sampling rate through the filtering is appreciably higher than symbol rate and providing that the rate of change of the dynamic multipath does not exceed the slewing rate of the decision-feedback loop,
However, these decision-feedback methods tend to be unacceptably slow in converging the equalization and echo-cancellation filtering to nearly optimal response when initially receiving a DTV signal that has bad multipath distortion. Bad multipath distortion conditions include cases where echoes of substantial energy lead or lag the principal received signal by more than ten or twenty microseconds, cases where there is an ensemble of many echoes with differing timings relative to the principal received signal, cases where multipath distortion changes rapidly, and cases where it is difficult to distinguish principal received signal from echo(es) because of similarity in energy level.
Worse yet, convergence is too slow when tracking of dynamic multipath conditions must be regained after the slewing rate of the decision-feedback loop has not been fast enough to keep up with rapid change in the multipath conditions. Data dependent equalization and echo cancellation methods that provide faster convergence than LMS or block-LMS decision-feedback methods are known, but there is difficulty in implementing them in an integrated circuit of reasonable size.
Accordingly, it is desirable to modify A/53 DTV signal to introduce periodically an ECR signal that will xe2x80x9cinstantlyxe2x80x9d converge the equalization and echo-cancellation filtering to substantially optimally response. It would be desirable to have an ECR signal that does not interfere with the operation of DTV signal receivers already in the field. However, because of the de-interleaving of VSB-8 signals in the DTV receiver, this is probably an impossible condition to satisfy, at least completely.
U.S. patent application Ser. No. 09/765,019 filed by A. L. R. Limberg on Jan. 18, 2001 and titled xe2x80x9cGHOST CANCELLATION REFERENCE SIGNALS FOR BROADCAST DIGITAL TELEVISION SIGNAL RECEIVERS AND RECEIVERS FOR UTILIZING THEMxe2x80x9d was published Oct. 25, 2001 as U.S. patent application Ser. No. 2001-0033341Kind Code A1. It describes each data field being extended a prescribed number of data segments to permit the inclusion of ECR signals composed of repetitive-PN511 sequences with baud-rate symbols. Application Ser. No. 20010033341 also specified that the precode signal repeat the last twelve symbols of the 313th data segment, just as in the standard VSB-8 DTV signal. Extension of the data field to include more than 313 data segments minimizes the modifications of the convolutional interleaver in the DTV transmitter and of the corresponding de-interleaver in the DTV receiver that would have to be made in newly designed DTV receivers. However, the extended data fields will interfere with the operation of some receivers already in the field.
Application Ser. No. 200010033341 points out that ECR signal should have sufficient energy that match filtering using auto-correlation procedures can distinguish the longest delayed echoes of the ECR signal from interference caused by other signals and by noise. Accordingly, ECR signals with substantial energy and well-defined auto-correlation responses are a desideratum. The triple PN63 sequence in the initial data segment of each data field of an A/53 broadcast DTV signal has a well-defined auto-correlation response, but has insufficient energy for detecting longer-delayed post-echoes with smaller amplitudes. The PN511 sequence in the initial data segment of each data field of an A/53 broadcast DTV signal has substantial energy and a well-defined auto-correlation response. However, no component sequence of the data field synchronizing (DFS) signal or combination of its component sequences has proven in practice to be very satisfactory as an ECR signal.
One reason is that no portion of the DFS signal is preceded by an information-free interval of sufficient duration that post-echoes of previous data and data segment synchronizing sequences exhibit insignificant spectral energy during the duration of that portion of the DFS signal to be used as ECR signal. Also, the A/53 DTV signals do not provide for the generation of an information-free interval such duration before the ECR signal by combining information sent at different times, a technique used in de-ghosting NTSC analog television signals. A 60-microsecond-long information-free interval extending over 646 symbol epochs should precede the ECR signal if it is not to be overlapped by the post-echoes of previous signals, which post-echoes can have significant energy if delayed no more than sixty microseconds or so. The post-echoes of previous signals should be kept from contributing significantly to digitized Johnson noise, in order to preserve the sensitivity of echo detection. Similarly, no portion of the DFS signal is succeeded by an information-free interval of sufficient duration that pre-echoes of subsequent data and data segment synchronizing sequences exhibit insignificant spectral energy during the duration of that portion of the DFS signal to be used as ECR signal. A 30-microsecond-long information-free interval extending over 323 symbol epochs should succeed the ECR signal if it is not be overlapped by the pre-echoes of previous signals, which pre-echoed can have significant energy if advanced no more than thirty microseconds or so. These information-free intervals preferably should be of even longer durations if auto-correlation filtering employing linear convolution is to be used for echo detection.
Another reason the PN511 sequence in the initial data segment of each data field of an ATSC broadcast DTV signal is not particularly satisfactory as an ECR signal is that the PN511 sequence is not repetitive. Therefore, the auto-correlation properties of the PN511 sequence are compromised. The reader is referred to U.S. Pat. No 5,065,242 titled xe2x80x9cDEGHOSITING APPARATUS USING PSEUDORANDOM SEQUENCESxe2x80x9d issued 23 Aug. 1994 to Charles Dietrich and Arthur Greenberg. This patent, incorporated herein by reference, pints out that the auto-correlation function of a maximal-length pseudorandom noise (PN) sequence has a cyclic, nature The patent describes repetitive PN sequences being inserted as ECR signal into a prescribed scan line interval of each of the vertical blanking intervals of NTSC analog television signals. U.S. Pat. No. 5,065,242 describes the transmission/reception channel characterization being performed using fast Fourier transform. (FFT) or discrete Fourier transform (DFT) methods.
In this specification and the claims appended thereto the phrase xe2x80x9crepetitive pseudo-random noise sequencexe2x80x9d is to be construed as being descriptive of a single continuous sequence, rather than as being descriptive of an intermittently repeated pseudo-random noise sequence. The cycle of a repetitive maximal-length PN sequence is defined in this specification and the claims appended thereto to extend over time until the xe2x80x9crandomxe2x80x9d pattern of binary values thereof begins to repeat. This definition is not at variance with common usage. The cycle of a repetitive maximal-length PN sequence is measured by the time between peaks of the autocorrelation function of the PN sequence.
The 90-microsecond of so possible range of echoes that is now known to exist in actual practice is appreciably more extensive than A. L. R. Limberg presumed when on 19 Jan. 2000 he filed provisional U.S. patent application Ser. No. 60/178,081, the priority document for U.S. patent application Ser. No. 09/765,019. Limberg presumed an echo range of only 45 microseconds or so, and the ECR signals specifically described relied on repetitive PN511 sequences with baud-rate symbols rendered as +5 or xe2x88x925 values. Limberg described the repetitive PN511 sequences being chosen such that they incorporated the |5, xe2x88x925, xe2x88x925, |5 symbol sequences at 832-symbol-epoch intervals, which sequences are used as data segment synchronizing (DSS) signals in DTV transmissions made in accordance with A/53. Baud-rate repetitive PN511 sequences are capable of unambiguous detection of echoes over a range of less than 47.5 microseconds.
In spring 2000, when it was reported to-the ATSC Task Force on RF System. Performance that the range of echoes with significant energy apt to be encountered in the field could be 90 microseconds or so wide, A. L. R. Limberg realized that unambiguous detection of echoes over so wide a range would be facilitated by ECR signals that employed baud-rate repetitive PN1023 sequences. The question was whether repetitive PN1023 sequences existed that incorporated the +5, xe2x88x925, xe2x88x925, +5 DSS sequences at four consecutive 832-symbol-epoch intervals. While he doubted that such repetitive PN1023 sequence existed, A. L. R. Limberg asked this question of the ATSC Task Force on. RF System Performance using e-mail, indicating he did not have the software for calculating all PN1023 sequences, replicating them and sifting the results.
Surprisingly, D. J. McDonald replied via e-mail later the same day that certain repetitive PN1023 sequences did indeed meet this criterion and that a number of others incorporated the +5, xe2x88x925, xe2x88x925, +5 DSS sequences at a lesser number of consecutive 832-symbol-epoch intervals. D. J. McDonald found sequences of the type desired by writing a program for sifting through an already existent file found on line. More DSS sequences have to be subsumed by a repetitive-PN sequence as n increases beyond 8 or so, but this problem is not as difficult as it first appears. As the (Pnxe2x88x921) length of a pseudo-random noise (PN) sequence increases with increase in the number n, the number of sequences increases more than linearly.
Further aspects of the invention concerned exactly how to incorporate the 3096-symbol-epoch triple-PN1023 sequence into the ATSC standard broadcast signal inasmuch as more than three data segments would be required to contain the entire 3096 symbol sequence. C. B. Patel suggested that the DFS signal be modified, eliminating the PN511 sequence and the initial PNT63 sequence to leave room for the tail of a 3096-symbol-epoch triple-PN1023 sequence beginning the third from final data segment of the previous data field. A. L. R. Limberg suggested that the DFS signal be modified, eliminating the PN511 sequence but retaining the initial PN63 sequence, and that the triple-PN1023 sequence be truncated to 3011 symbol epochs. This would still permit linear convolution of a PN1023 auto-correlation filter with the received repetitive-PN1023 sequence to detect without ambiguity echoes extending over a 90-microsecond range.
A. L. R. Limberg and C. B. Patel wanted to truncate the repetitive-PN1023 sequence even further to 2500 symbol epochs, so it could be fitted into three consecutive data segments. This would facilitate leaving the DFS signal in the ATSC standard intact, but would reduce to less than the desired 90 microseconds the range of echoes that could be detected without ambiguity by simply passing the received repetitive PN1023 sequence through a PN1023 auto-correlation filter in a simple linear convolution procedure. D. J. McDonald pointed out that the cyclic nature of the repetitive PN1023 sequence meant that all the echo information required for DFT procedures for characterizing the channel reposed in a interior cycle of the PN1023 sequence overlapped only by echoes of itself and flanking PN1023 signal. This permits DFT procedures to detect echoes without ambiguity over an echo range approaching 95 microseconds width, so long as there are at least two cycles of PN1023 sequence in the ECR signal. The interior cycle of the PN1023 sequence can be looped back on itself to extend the sequence in length for calculation purposes.
When A. L. R. Limberg relayed this observation to C. B. Patel, Dr. Patel discerned that looping an interior cycle of the PN1023 sequence back on itself permitted circular convolution with the kernel of a PN1023 auto-correlation filter, for detecting echoes without ambiguity over an echo range approaching 95 microseconds width, so long as there are at least two cycles of PN1023 sequence in the ECR signal
Aspects of the invention concern incorporating echo-cancellation reference (ECR) signals into a DTV signal with a symbol rate of around 10.76 million samples per second, wherein each of the ECR signals includes or essentially consists of a repetitive PN-1023 sequence with baud-rate symbols rendered as +5 or xe2x88x925 values, which repetitive-PN1023 sequence incorporates a number of consecutive data-segment-synchronization signals. Other aspects of the invention concern transmitters and receivers for such signals.